how does the cell wall aid in classifying bacteria ?
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Jacob M. Taylor
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The cell wall is the outermost component common to all bacteria (except Mycoplasma species, which are bounded by a cell membrane, not a cell wall). The cell wall is located external to the cytoplasmic membrane and is composed of peptidoglycan. The peptidoglycan provides structural support and maintains the characteristic shape of the cell.
Peptidoglycan
Peptidoglycan is a complex, interwoven network that surrounds the entire cell and is composed of a single covalently linked macromolecule. It is found only in bacterial Cell walls. It provides rigid support for the cell, is important in maintaining the characteristic shape of the cell, and allows the cell to withstand media of low osmotic pressure, such as water. The term peptidoglycan is derived from the peptides and the sugars (glycan) the make up the molecule. Synonyms for peptidoglycan are murein and mucopeptide.
Attached to each of the muramic acid molecules is a tetrapeptide consisting of both D- and L-amino acids, the precise composition of which differs from one bacterium to another. Two of these amino acids are worthy of special mention: diaminopimelic acid, which is unique to bacterial cell walls, and d-alanine, which is involved in the cross-links between the tetrapeptides and in the action of penicillin. Note that this tetrapeptide contains the rare d-isomers of amino acids; most proteins contain the l-isomer. The other important component in this network is the peptide cross-link between the two tetrapeptides. The cross-links vary among species; in staphylococcus aureus, for example, five glycines link the terminal D-alanine to the penultimate L-lysine. Because peptidoglycan is present in bacteria but not in human cells, it is a good target for antibacterial drugs. Several of these drugs, such as penicillins, cephalosporins, and Vancomycin, inhibit the synthesis of peptidoglycan by inhibiting the transpeptidase that makes the cross-links. Between the two adjacent tetrapeptides lysozyme, an enzyme present in human tears, mucus, and saliva, can cleave the peptidoglycan backbone by breaking its glycosyl bonds, thereby contributing to the natural resistance of the host to microbial infection. Lysozyme-treated bacteria may swell and rupture as a result of the entry of water into the cells, which have a high internal osmotic pressure. However, if the lysozyme-treated cells are in a solution with the same osmotic pressure as that of the bacterial interior, they will survive as spherical forms, called protoplasts, surrounded only by a cytoplasmic membrane.
Lipopolysaccharide
The lipopolysaccharide (LPS) of the outer membrane of the cell wall of gram-negative bacteria is endotoxin. It is responsible for many of the features of disease, such as fever and shock (especially hypotension), caused by these organisms . It is called endotoxin because it is an integral Part of the cell wall, in contrast to exotoxins, which are actively secreted from the bacteria. The constellation of symptoms caused by the endotoxin of one gram-negative Bacterium is similar to another, but the severity of the symptoms can differ greatly. In contrast, the symptoms caused by exotoxins of different bacteria are usually quite different.
The LPS is composed of three distinct units:
(1) A phospholipid called lipid A, which is responsible for the toxic effects.
(2) A core polysaccharide of five sugars linked through Ketodeoxyoctulonate (KDO) to lipid A.
(3) An outer polysaccharide consisting of up to 25 repeating units of three to five sugars. This outer polymer is the important somatic, or O, antigen of several gram-Negative bacteria that is used to identify certain organisms
In the clinical laboratory. Some bacteria, notably members of the genus Neisseria, have an outer lipooligosaccharide (LOS) containing very few repeating units of sugars.
Teichoic Acid
Teichoic acids are fibers located in the outer layer of the Gram-positive cell wall and extend from it. They are composed of polymers of either glycerol phosphate or ribitol Phosphate. Some polymers of glycerol teichoic acid penetrate the peptidoglycan layer and are covalently linked to the lipid in the cytoplasmic membrane, in which case they are called lipoteichoic acid; others anchor to the muramic Acid of the peptidoglycan.
The medical importance of teichoic acids lies in their ability to induce inflammation and septic shock when caused by certain gram-positive bacteria; that is, they activate the same pathways as does endotoxin (LPS) in Gram-negative bacteria. Teichoic acids also mediate the Attachment of staphylococci to mucosal cells. Gram-negative bacteria do not have teichoic acid
Difference
Gram positive bacteria have thick wall of peptidoglycan. Their cell wall is composed of peptidoglycan, lipopolysaccharides and teichoic acid. They have multilayers of peptidoglycan. The cell wall thickness is 100-120 Å. It is single layered that have low lipid content, whereas murein content is 70-80%.
Gram negative bacteria have thin wall of peptidoglycan. Their cell wall is composed of peptidoglycan, lipopolysaccharides. They have high percentage of lipids in their cell wall. Teichoic acid is absent in their cell wall. They have single layer of peptidoglycan. The cell wall thickness is 70-120 Å. It is two layered cell wall that have 20-30% lipid content and is 10-20% murein content.